Process for double platen densifying press

ABSTRACT

A method is disclosed for compressing thin, thermoplastic resincoated sheets of fibrous material employing a pair of platen assemblies each comprised of a plurality of platen members between which a plurality of sheets to be compressed are disposed and which are relatively displaceable toward one another to compress the sheet material therebetween. Relative displacement of the platen members to compress the sheet material therebetween is achieved in a press having opposed pressing members between which the platen sets are alternately disposed. The method includes loading sheet material into one of the sets of platen members while the latter set is disposed outside the press and the other set is disposed between the pressing members of the press, simultaneously moving the one set of platen members into the press and between the pressing members thereof and the other set of platen members out of the opposite side of the press, closing the pressing members relative to one another whereby the sheet material in the one platen assembly is compressed into the other platen assembly outside the press, simultaneously transferring the one platen assembly back to its initial position outside the press and moving the other platen assembly which is now loaded into the press, closing the pressing members whereby the sheet material in the other platen assembly is compressed, unloading the compressed sheet material from the one platen assembly, and then loading sheets of material to be compressed into the one platen assembly and repeating the alternate loading, pressing and unloading operations of the two platen assemblies. The method further includes heating the platen members of each of the assemblies during loading thereof to melt the resin with which the fiber material is coated, and cooling the platen members of each assembly during the pressing operation to cure the resin.

United States Patent Hubert et al.

[ PROCESS FOR DOUBLE PLATEN DENSIFYING PRESS Inventors: Joseph B. Hubert; Bernard K. Hook,

both of Hastings, Mich.

Assignee: Gulf & Western Industrial Products Company, Grand Rapids, Mich.

Filed: Jan. 28, 1972 Appl. No.: 221,636

US. Cl 100/38, 100/196, 100/224 Int. Cl 1330b l/32, B30b 13/00 Field of Search 100/38, 237, 221,

References Cited UNITED STATES PATENTS 6/1876 Boomer 100/224 7/1967 Cecchi 100/196 Primary Examiner-Leon G. Machlin Attorney-James H. Tilberry et a].

[4 1 Aug. 14, 1973 the sheet material therebetween. Relative displacement of the platen members to compress the sheet material therebetween is achieved in a press having opposed pressing members between which the platen sets are alternately disposed. The method includes loading sheet material into one of the sets of platen members while the latter set is disposed outside the press and the other set is disposed between the pressing members of the press, simultaneously moving the one set of platen members into the press and between the pressing members thereof and the other set of platen members out of the opposite side of the press, closing the pressing members relative to one another whereby the sheet-material in the one platen assembly is compressed into the other platen assembly outside the press, simultaneously transferring the one platen assembly back to its initial position outside the press and moving the other platen assembly which is now loaded into the press, closing the pressing members whereby the sheet material in the other platen assembly is compressed, unloading the compressed sheet material from the one platen assembly, and then loading sheets of material to be compressed into the one platen assembly and repeating the alternate loading, pressing and unloading operations of the two platen assemblies. The method further includes heating the platen members of each of the assemblies during loading thereof to melt the resin with which the fiber material is coated, and cooling the platen members of each assembly during the pressing operation to cure the resin.

8 Claims, 5 Drawing Figures PATENTEDAus 14 1915 3; 752.060

SMEEI 2 OF 2 26 FIG. 3

PROCESS FOR DOUBLE PLATEN DENSIFYING PRESS The present invention relates to the art of compressing fibrous sheet material and, more particularly, to a method for increasing the rate at which compressed fibrous sheet material can be produced from uncompressed material blanks.

Resin-coated fibrous sheet material has many uses in industry and, for example, is employed in making disposable containers for liquid products such as milk and oil. One of the factors determining the acceptance of such sheet material is the cost thereof, and the cost is in turn determined in part by the rate at which the sheet material can be produced. Such sheet material may, for example, be in the form of a fibrous paperboard blank which is sprayed or otherwise coated with a plastic resin for the purpose of rendering the paperboard material impervious to liquids. The plastic resin may be a thermoplastic resin such as a styrene, for example, and the coated blank is compressed under heat and pressure to cause impregnation of the fibrous sheet with resin and compression of the sheet to densify the fibrous material and reduce the transverse thickness thereof to a desired dimension.

Heretofore, blanks or continuous sheets of resincoated fibrous material have been fed into a press from one side thereof and between platen members of the press. The press is then actuated through a stroke of operation which displaces the platen members in a direction to compress the material therebetween and then returns the platens to their open positions. The compressed material is then discharged from the opposite side of the press, additional uncompressed material is introduced into the press from the one side thereof, and the above pressing procedure repeated. It will be appreciated that the foregoing procedure is both timeconsuming and expensive, even if provisions are made for simultaneously compressing more than one sheet for each working stroke of the press. In this respect, during the working stroke of the press, the feeding and discharge assemblies are dormant, whereby the pressing of the sheet material in the press accounts for the total work done during this stage of operation. When percent when the sheets compressed are of the same' material and same size as the sheets compressed in accordance with the method described above.

The foregoing reduction in time and cost per sheet is achieved in accordance with the present invention by alternately compressing a plurality of sheets of fibrous material at a common pressing station by means of platen sets or assemblies each comprised of three or more platen members, and by providing for one of the sets outside the pressing station to be loaded and unloaded while the other set is being acted upon at the pressing station to compress the sheet material therein. Preferably, the platen sets are supported on a common carriage reciprocable relative to the pressing station to position each set alternately at the pressing station and at a loading and unloading station adjacent the pressing station. Each platen set may, for example, be capable of receiving ten sheets of material to be compressed, whereby each complete cycle of loading and unloading the two sets and compressing the material therein results in the production of 20 sheets of compressed material. It will be appreciated that the complete cycle provides for a reduction of time in producing a given quantity of compressed sheets over the method heretofore employed by providing for compressed sheets to be discharged and uncompressed sheets to be loaded into a platen set during the time in which the sheets previously loaded into the other platen set are being compressed.

In accordance with yet another aspect of the method of the present invention, time can be saved by providing for the loading and discharge of a platen set to have a particular directional orientation relative to the path of movement of the platen sets between the loading and unloading station and the compressing station. Preferably, the loading and unloading operations are achieved by moving the sheet material in a direction generally perpendicular to the direction of the path of movement of the platen sets relative the compressing station. Further, it is preferred that loading of the platen sets be achieved from one side thereof and that unloading be achieved from the opposite thereof to enhance continuity of material handling during the loading and unloading operations. Such continuity and subsequent handling of sheets which have been compressed can be achieved by providing for loading of the two platen sets and the unloading thereof to be achieved from corresponding sides thereof.

It is accordingly an outstanding object of the present invention to provide a method of producing sheets of compressed fibrous material in greater quantities during a given period of time than heretofore possible.

Yet another object of the present invention is the provision of a method of producing sheets of compressed fibrous material which substantially reduces the per-sheet cost of production.

Still another object of the present invention is the provision of a method of producing sheets of compressed fibrous material by which the handling of the sheet material is achieved with maximum efficiency.

Still another object of the present invention is the provision of a method of producing compressed sheets of fibrous material by which a minimum amount of lost production time is achieved.

The foregoing objects, and others, will in part be obvious and in part more fully pointed outhereinafter in conjunction with the description of the drawing in which:

FIG. 1 is a plan view of a press arrangement by which the method of the present invention may be practiced;

FIG. 2 is a side elevation view, partially in section, of the press arrangement illustrated in FIG. 1, the view being along line 2-2 in FIG. 1;

FIG. 3 is an end elevation view, partially in section, taken along line 3-3 in FIG. 2 and illustrating a platen set at the compressing station prior to the compressing operation;

FIG. 3A is an end elevation view similar to FIG. 3, taken along line 3A--3A in FIG. 2, and illustrating platen set disposition during a compressing operation; and

FIG. 4 is an end elevation view taken along line 4-4 in FIG. 2.

Referring now to the drawings in greater detail wherein the showings are for the purpose of illustrating one form of apparatus by which the method of the present invention may be practiced. A press is illustrated which has an opening 12 extending therethrough from one side of the press to the other. Opening 12 permits carriage means 14 associated with the press to translate back and forth along a path parallel to the axis of opening 12. Carriage means 14 is of a length relative to the length of opening 12 to provide for the opposite ends 16 and 18 thereof to be disposed alternately generally centrally of the length of opening 12 and longitudinally outwardly of the corresponding side of the press. Upon movement of carriage means 14 to the left in FIG. 1, for example, end 18 moves into a position generally centrally of the length of opening 12 and end 16 moves longitudinally outwardly of the corresponding end of press 10 to the broken line position thereof illustrated in FIGS. 1 and 2.

Carriage means 14 may be of any suitable structure and may be supported for translating movement relative to press 10 in any suitable manner. In the arrangement illustrated, carriage means 14 includes an open frame portion 20 having wheels 22 at opposite ends thereof by which the carriage is supported for rolling movement along a pair of track components 24 extending longitudinally through opening 12. It will be appreciated that the carriage could, for example, be slidably supported by suitable guideway means, or otherwise supported for reciprocating movement relative to press 10. Reciprocating movement may be imparted to carriage means 14 in any suitable manner and could, for example, be achieved manually or by suitable motor means such as a hydraulic or pneumatic piston and cylinder motor arrangement operatively interconnected with the carriage.

Platen sets 26 and 28 are supported on carriage means 14 adjacent the opposite ends thereof. Each platen set is comprised of a plurality of platen members structurally interrelated to be relatively displacable toward and away from one another to provide for introducing sheet material therebetween and compressing the sheet material placed therebetween. With reference to platen set 28, for example, each of the platen sets includes a plurality of platen members 30 in the form of metallic plate elements disposed one above the other in substantially parallel planes. Any number of platen members may be provided in the sets, and in the arrangement illustrated each platen set includes 10 platen members. The platen members of a set may be structurally interrelated in any desired manner to provide for the members to be relatively displacable to permit introduction and removal of sheet material relative thereto and to provide for sheet material placed therebetween to be compressed. For example, the corresponding ends of adjacent platen members may be interconnected by link assemblies 32 comprised of link members pivotally interconnected with the platen members and with one another so that the several platen members are expandable and contractible in a direction generally perpendicular to the planes thereof.

The linkage assemblies keep the platen members longitudinally and laterally aligned relative to one another during relative movement therebetween. The manner in which the platen members are moved relative to one another to separate the platen members to facilitate the introduction and removal of material therefrom is not important to the present invention, and such movement can be achieved in any one of a number of suitable ways. For example, means could be provided adjacent the corresponding press end to engage and lift the uppermost platen member which would result in opening the spaces between the platen members. As another example, mechanical wedge means could be introduced between the opposed faces of adjacent platen members to cause separation thereof.

The specific structure of press 10 is not pertinent to the present invention, but it will be appreciated that the press includes a pair of opposed pressing members relatively displacable toward and away from one another during a press stroke to perform a compressing operation. The manner in which the pressing members are relatively displaced and the means by which such displacement is achieved is readily variable. For example, the pressing members could be hydraulically, pneumatically or mechanically actuated, and one or the other or both of the pressing members could be movable relative to the press frame during a compressing operation. Accordingly, it will be appreciated that the following description of the press depicted in the drawings is merely illustrative of one form of press which could be employed in practicing the present invention. In the arrangement illustrated, the press includes a fixed upper pressing member 34 and a reciprocable lower pressing member 36 adapted to be raised and lowered relative to member 34. Suitable hydraulic drive means 38 is provided to impart vertical upward movement to pressing member 36 and it will be appreciated that the hydraulic drive means receives hydraulic fluid from a suitable source, not illustrated, and that the hydraulic fluid pressure is sufficient to achieve the desired compressing operation. When carriage means 14 is in the position illustrated in FIG. 2, the lower platen member 30 of platen set 26 overlies lower pressing member 36 and the upper platen member 30 underlies pressing member 34, and the platen set is positioned generally symmetrically with regard to a vertical axis through the pressing members. Preferably, plate components 40 and 42 are disposed respectively above and below the upper and lower platen members so that the pressing members 34 and 36 do not directly engage the latter platen members. Lower plate 42 rests on the opposite sides of open frame 20 of carriage means 14, and the open frame structure permits pressing member 36 to move upwardly inside the frame of the carriage to engage the underside of plate 42. Upon further upward movement of pressing member 36, platen set 26 is elevated to the position illustrated in FIG. 3A, whereby platen members 30 are relatively displaced toward one another to compress sheets of material S disposed therebetween. Following the compressing operation, lower pressing member 36 descends whereby platen set 26 is lowered onto carriage means 14 in preparation for movement of the platen set to its loading and unloading position outside the press.

During the compressing operation described above with regard to platen set 26, platen set 28 is disposed outwardly of the corresponding side of press 10 and sheet material therein which has been compressed is removed therefrom and sheets of material to be compressed are introduced thereinto. It will be appreciated, of course, that platen members 30 of platen set 28 are moved relative to one another in a direction to separate adjacent platen members to facilitate the unloading and loading thereof. Preferably, compressed sheet material is unloaded from the platen set from one side thereof and in a direction such as that indicated by arrows A in FIG. 4, and material to be compressed is introduced between the platen members from the opposite side of the platen set and in a direction such as that indicated by arrows B in FIG. 4. In this manner, material loading and discharging is achieved in the least amount of time. It will be appreciated, however, that unloading and loading could both be achieved from one side of the platen set and that in such case either side could be employed.

With the foregoing description in mind, it will be seen with reference to FIG. 1 of the drawing that press defines a compressing station C, a loading and unloading station D is established adjacent one side of the press for platen set 26, and that a loading and unloading station E is established adjacent the opposite side of the press for platen set 28. It will be appreciated that the loading and unloading of a platen set at station D or E can be achieved manually or by suitable loading and unloading mechanisms such as schematically illustrated respectively by numerals 44 and 46 in FIG. 1. Preferably, as mentioned hereinabove, loading and unloading whether achieved manually or otherwise is performed on opposite sides of a platen set disposed at its loading and unloading station, and in the arrangement illustrated in FIG. 1 unloading is in the direction of arrows A and loading is in the direction of arrows B. This arrangement advantageously facilitates material handling and the flow of material to and from the compressing station. Moreover, material handling is further facilitated by providing for the flow of material during loading and unloading to be along aligned paths on opposite sides of the platen set. In other words, the paths defined by arrows A and B for a given platen set coinloading and unloading paths extend substantially perpendicular to the path of reciprocating movement of the carriage means for the platen sets. This arrangement provides for the flow of material during loading, compressing and unloading to be achieved in a minimum amount of floor space for a particular structural arrangement being employed.

Presuming the components of the arrangement illustrated to be in the positions thereof depicted in FIG. 1, and presuming platen set 26 disposed at pressing station C to be loaded with sheet material to be compressed, it will be appreciated in view of the foregoing description, that the following sequence of operations takes place. Press 10 is actuated for pressing member 36 thereof to elevate platen set 26 to achieve compression of the sheet material loaded therein. While the latter compressing operation is taking place sheet material to be compressed is loaded into platen set 28 and between the platen members thereof in the direction indicated by arrow B either by suitable mechanical loading means or by hand. When the compressing oper-- ation is completed, pressing member 36 is lowered and platen set 26 is returned to carriage 14. The carriage is then translated to the left in FIG. 1, whereby platen ascide longitudinally. It is further preferred that the linear sembly 28 is moved into compressing station C and platen set 26 is returned to its loading and unloading station D. The press is again actuated to achieve compression of the sheet material in platen set 28, and while the pressing operation is taking place, the platen members of platen set 26 are opened, the compressed sheet material is removed therefrom in the direction of arrow A either manually or by suitable mechanical unloading means 46, and material to be compressed is introduced between the platen members of platen set 26 in the direction of arrow B and either manually or by suitable mechanical loading means 44. When the pressing operation is completed, carriage 14 is translated to the right in FIG. 1 to move platen assemblies 26 and 28 to the solid line positions thereof illustrated in FIG. 1. The sheet material in platen set 26 is then compressed in the foregoing manner, the platen members of platen set 28 are separated to facilitate the removal of compressed sheetmaterial therefrom in the direction of arrow A and the loading of sheet material to be compressed thereinto in the direction of arrow B. The foregoing sequence of operations is then repeated.

As mentioned hereinabove, the sheet material being compressed is coated with a film or layer of a thermoplastic resin. To achieve compression of the sheet material and impregnation thereof by the thermoplastic resin, it becomes necessary to heat the resin to elevate the temperature thereof to the melting point for the particular resin employed. Moreover, it may be desirable to positively cool the compressed sheet material and thus assure curing or substantial curing of the resin prior to discharge of the compressed sheet material from a platen set. Accordingly, it is contemplated that the platen members of the platen sets will be heated while the material is being loaded therein and during transfer thereof to the pressing station. Such heating may be achieved in any number of ways and, for example, the platen members may be of hollow construction or provided with passageways extending therethrough parallel to the opposite faces thereof which engage the material to be compressed. A heating fluid, such as steam, can be circulated through the platen members during the loading operation and during transfer thereof from the loading and unloading station to the compressing station. The heating fluid can be delivered to the platen members through flexible hose means to permit movement of the platen members relative to the source of heating fluid. Cooling of the platen members of a platen set following the compressing operation preferably is initiated while the platen set is at the compressing station. Such cooling can be achieved, for example, by circulating a cooling fluid such as water or a suitable refrigerant through the platen members. For this purpose, the same flexible hoses can be employed as are employed to circulate the heating fluid in which case it would be necessary to provide suitable controls such as valves to shut off the flow of one of the heating and cooling fluids while the other is being circulated. Alternatively, separate flow lines could be provided for the cooling fluid. Further, it will be appreciated that heating could be achieved by means other than fluid circulation such as, for example, by electrical resistance heating elements in or about the platen members.

The length of time during which the platen members are heated and cooled will depend, of course, on many factors including the properties of the particular resin employed, the thermal conductivity of the metal material of the platen members, the area of the platen members to be heated or cooled and the magnitudes of the heating and cooling temperatures. Similarly, the pressure employed to achieve compression of the sheet material to reduce the thickness thereof to a desired dimension is variable and dependent in part on the density of the uncompressed sheet material and the extent of reduction of thickness desired. All of these foregoing variables will of course be considered and the method of the present invention will be performed in a manner whereby the end results sought are achieved in the least amount of time possible. In any event, the method can be performed in a manner whereby the loading and unloading of a platen set is achieved in substantially the same amount of time that it takes to compress and cool material at the pressing station, whereby the only portion of operation during which no work is performed is during translation of the platen sets toand from the pressing station.

It will be appreciated that the press apparatus could be designed to support the carriage in the press against vertically downward displacement relative thereto and that the press could be provided with a movable upper pressing member which would press downwardly on the platen set to achieve material compression. Moreover, it will be appreciated that the platen sets could be supported by separate carriage assemblies independently reciprocable between the corresponding platen loading and unloading station and the compression station and that the individual carriage assemblies could be provided with corresponding actuating means for imparting reciprocating movement thereto. In such case, the two carriage assemblies might be reciprocated in unison or otherwise to alternately position the two platen sets at the compressing station. It will be further appreciated that while the platen members of the platen sets are illustrated as having substantially planar surfaces between which the material to be compressed is disposed, the opposed faces could be otherwise cooperatively contoured for compressing sheet material dis-' posed therebetween. For example, the opposed faces of the platen members could be provided with cooperative corrugated contours to achieve compression of sheet material from a flat to a corrugated crosssectional configuration. Moreover, while the method of the present invention has been described in conjunction with the compressing of thermoplastic resin coated paperboard sheet material, it will be appreciated that the method can be employed in comprising other fibrous sheet materials which may not be resin coated or impregnated.

As the method of the present invention may be practiced employing structural arrangements other than that disclosed herein and as many possible changes can be made in the structural arrangements illustrated herein and the method of the present invention still practiced, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the practice of the present invention and not as a limitation.

Having thus described our invention, we claim:

1. A method of compressing sheets of fibrous material between sets of opposed platen members relatively displaceable toward and away from one another comprising: providing a compressing station, establishing a first platen station for one of the sets of platen members and a second platen station for the other of the sets of platen members, loading sheet material between the platen members of the one set at said first station, transferring said one set of platen members to said compressing station, relatively displacing the platen members of said one set at said compressing station to compress the sheet material therebetween, transferring said one set back to said first station, removing the compressed sheet material from between the platen members of said one set at said first station, loading sheet material between the platen members of said other set at said second station, transferring said other set of platen members to said compressing station, relatively displacing the platen members of said other set at said com-pressing station to compress the sheet material therebetween, transferring said other set back to said second station, and removing the compressed sheet material from between the platen members of said other set at said second station, heating the platen members of said one set and said other set at the respective one of said first and second stations prior to transferring said one and said other set from the respective first and second station to said compressing station, and cooling the platen members of said one and said other set at said compressing station.

2. The method of claim 1, and transferring said other set of platen members from said second station to said compressing station simutaneous with the transfer of said one set of platen members from said compressing station to said first station.

3. The method of claim 1, and loading said other set of platen members while said one set of platen members is at said compressing station.

4. The method of claim 3, and removing said compressed sheet material from between the platen members of said one set while said other set of platen members is at said compressing station.

5. The method of claim 4, and loading sheet material between and removing compressed sheet material from the platen members of each of said one and said other sets along corresponding feed and discharge paths having a common direction.

6. The method of claim 5, and transferring said one and said other sets of platen members to and from said compressing station along aligned paths on opposite sides of said compressing station.

7. The method of claim 6, and providing for said feed and discharge paths of each of said one and said other sets of platen members to be generally perpendicular to said aligned paths and inthe same direction relative thereto.

8. A method of compressing sheets of fibrous material between sets of opposed platen members relatively displaceable toward and away from one another comprising: providing a compressing station, establishing a first platen station for one of the sets of platen members and a second platen station for the other of the sets of platen members, loading sheet material between the platen members of the one set at said first station, transferring said one set of platen members to said compressing station, relatively displacing the platen members of said one set at said compressing station to compress the sheet material therebetween, transferring said one set back to said first station, removing the compressed sheet material from between the platen members of said one set at said first station, loading sheet material between the platen memers of said other set at said second, station, transferring said other set of platen members to said compressing station, relatively displacing the platen members of said other set at said compressing station to compress the sheet material therebetween, transferring said other set back to said M second station, removing the compressed sheet matethe platen members of each of said'one and said other sets along corresponding feed and discharge paths having a common direction, transferring said one and said other sets of platen members to and from said compressing station along aligned paths on opposite sides of said compressing station, providing for said feed and discharge paths of each of said one and-said other sets of platen members to be generally perpendicular to said aligned paths and in the same direction relative thereto, heating the platen members of said one and said other set when said sets are at the respective first and second stations for loading, and cooling the platen members of said one and said other set when said sets 

1. A method of compressing sheets of fibrous material between sets of opposed platen members relatively displaceable toward and away from one another comprising: providing a compressing station, establishing a first platen station for one of the sets of platen members and a second platen station for the other of the sets of platen members, loading sheet material between the platen members of the one set at said first station, transferring said one set of platen members to said compressing station, relatively displacing the platen members of said one set at said compressing station to compress the sheet material therebetween, transferring said one set back to said first station, removing the compressed sheet material from between the platen members of said one set at said first station, loading sheet material between the platen members of said other set at said second station, transferring said other set of platen members to said compressing station, relatively displacing the platen members of said other set at said com-pressing station to compress the sheet material therebetween, transferring said other set back to said second station, and removing the compressed sheet material from between the platen members of said other set at said second station, heating the platen members of said one set and said other set at the respective one of said first and second stations prior to transferring said one and said other set from the respective first and second station to said compressing station, and cooling the platEn members of said one and said other set at said compressing station.
 2. The method of claim 1, and transferring said other set of platen members from said second station to said compressing station simutaneous with the transfer of said one set of platen members from said compressing station to said first station.
 3. The method of claim 1, and loading said other set of platen members while said one set of platen members is at said compressing station.
 4. The method of claim 3, and removing said compressed sheet material from between the platen members of said one set while said other set of platen members is at said compressing station.
 5. The method of claim 4, and loading sheet material between and removing compressed sheet material from the platen members of each of said one and said other sets along corresponding feed and discharge paths having a common direction.
 6. The method of claim 5, and transferring said one and said other sets of platen members to and from said compressing station along aligned paths on opposite sides of said compressing station.
 7. The method of claim 6, and providing for said feed and discharge paths of each of said one and said other sets of platen members to be generally perpendicular to said aligned paths and in the same direction relative thereto.
 8. A method of compressing sheets of fibrous material between sets of opposed platen members relatively displaceable toward and away from one another comprising: providing a compressing station, establishing a first platen station for one of the sets of platen members and a second platen station for the other of the sets of platen members, loading sheet material between the platen members of the one set at said first station, transferring said one set of platen members to said compressing station, relatively displacing the platen members of said one set at said compressing station to compress the sheet material therebetween, transferring said one set back to said first station, removing the compressed sheet material from between the platen members of said one set at said first station, loading sheet material between the platen memers of said other set at said second station, transferring said other set of platen members to said compressing station, relatively displacing the platen members of said other set at said compressing station to compress the sheet material therebetween, transferring said other set back to said second station, removing the compressed sheet material from between the platen members of said other set at said second station, transferring said other set of platen members from said second station to said compressing station simultaneous with the transfer of said one set of platen members from said compressing station to said first station, removing said compressed sheet material from between the platen members of said one set while said other set of platen members is at said compressing station, loading sheet material between and removing compressed sheet material from the platen members of each of said one and said other sets along corresponding feed and discharge paths having a common direction, transferring said one and said other sets of platen members to and from said compressing station along aligned paths on opposite sides of said compressing station, providing for said feed and discharge paths of each of said one and said other sets of platen members to be generally perpendicular to said aligned paths and in the same direction relative thereto, heating the platen members of said one and said other set when said sets are at the respective first and second stations for loading, and cooling the platen members of said one and said other set when said sets are at said compressing station. 